The present invention relates to an improved method of depositing at least one layer of fine-crystalline xcex3-Al2O3 by reactive magnetron sputtering technique onto a cutting tool for metal machining.
It is well-known that for, e.g., cemented carbide cutting tools used in metal machining, the wear resistance of the tool edge can be increased considerably by applying thin, hard surface layers of metal oxides, carbides or nitrides with the metal either selected from the transition metals from the groups IV, V and VI of the Periodic Table or from silicon, boron and aluminum. The coating thickness usually varies between 1 and 15 xcexcm and the most widespread techniques for depositing such coatings are PVD and CVD (Chemical Vapor Deposition). It is also well-known that further improvements of the performance of a cutting tool can be achieved by applying a pure ceramic layer such as Al2O3 on top of layers of metal carbides and nitrides (U.S. Pat. Nos. 5,674,564 and 5,487,625).
Cemented carbide cutting tools coated with alumina layers have been commercially available for over two decades. The CVD technique usually employed involves the deposition of material from a reactive gas atmosphere on a substrate surface held at elevated temperatures. Al2O3 crystallizes into several different phase such as xcex1 (alpha), xcexa (kappa) and "khgr" (chi), called the xe2x80x9cxcex1-seriesxe2x80x9d with hcp (hexagonal close packing) stacking of the oxygen atoms, and into xcex3 (gamma), xcex8 (theta), xcex7 (eta) and xcex4 (delta), called the xe2x80x9cxcex3-seriesxe2x80x9d with fcc (face centered cubic) stacking of the oxygen atoms. The most often occurring Al2O3-phases in CVD coatings deposited on cemented carbides at conventional CVD temperatures, 1000xc2x0-1050xc2x0 C., are the stable alpha and the metastable kappa phases, however, occasionally the metastable theta phase has also been observed.
The CVD Al2O3-coatings of the xcex1-, xcexa- and/or xcex8-phase are fully crystalline with a grain size in the range of 0.5-54 xcexcm and having well-faceted grain structures.
The inherently high deposition temperature of about 1000xc2x0 C. renders the total stress in CVD Al2O3-coatings on cemented carbide substrates to be tensile, hence the total stress is dominated by thermal stresses caused by the difference in thermal expansion coefficients between the substrate and the coating and less by intrinsic stresses which have their origin from the deposition process itself and are of compressive nature. The tensile stresses may exceed the rupture limit of Al2O3 and cause the coating to crack extensively and thus degrade the performance of the cutting edge in, e.g., wet machining where the corrosive chemicals in the coolant fluid may exploit the cracks in the coating as diffusion paths.
Generally CVD-coated tools perform very well when machining various steels and cast irons under dry or wet cutting conditions. However, there exists a number of cutting operations or machining conditions when PVD-coated tools are more suitable, e.g., in drilling, parting and threading and other operations where sharp cutting edges are required. Such cutting operations are often referred to as the xe2x80x9cPVD coated tool application areaxe2x80x9d.
Plasma assisted CVD, PACVD, makes it possible to deposit coatings at lower substrate temperatures as compared to thermal CVD temperatures and thus avoid the dominance of the thermal stresses. Thin Al2O3 PACVD films, free of cracks, have been deposited on cemented carbides at substrate temperatures of 450-700xc2x0 C. (U.S. Pat. Nos. 5,516,588 and 5,587,233). The PACVD process for depositing Al2O3 includes the reaction between an Al-halogenide, e.g., AlCl3, and oxygen donor, e.g., CO2. Because of the incompleteness of this chemical reaction, chlorine is trapped in the Al2O3-coating and its content could be as large as 3.5%. Furthermore, these PACVD Al2O3-coatings are generally composed of, besides the crystalline alpha- and/or gamma-Al2O3-phase, a substantial amount of amorphous alumina which in combination with the high content of halogen impurities, degrades both the chemical and mechanical properties of said coating, hence making the coating material non-optimized as a tool material.
The field of the present invention relates particularly to the art of PVD Al2O3 coated cutting tools used in metal machining.
There exist several PVD techniques capable of producing refractory thin films on cutting tools and the most established methods are iron plating, DC- and RF-magnetron sputtering, arc discharge evaporation, IBAD (Ion Beam Assisted Deposition) and Activated Reactive Evaporation (ARE). Each method has its own merits and the intrinsic properties of the produced coatings such as microstructure/grainsize, hardness, state of stress, intrinsic cohesion to the underlying substrate may vary depending on the particular PVD method chosen. Early attempts to PVD deposit Al2O3 at typical PVD temperatures, 400-500xc2x0 C., resulted in amorphous alumina layers which did not offer any notable improvement in wear resistance when applied on cutting tools. PVD deposition by HF diode- or magnetron sputtering resulted in crystalline xcex1-Al203 only when the substrate temperature was kept as high as 1000xc2x0 C. (Thornton and Chin, Ceramic Bulletin, 56(1977)504). Likewise, applying the ARE method for depositing Al2O3, only resulted in fully dense and hard Al2O3-coatings at substrate temperatures around 1000xc2x0 C. (Bunshah and Schramm, Thin Solid Films, 40(1977)211).
With the invention of the bipolar pulsed DMS technique (Dual Magnetron Sputtering), which is disclosed in DD 252 205 and U.S. Pat. No. 5,698,314, a wide range of opportunities opened up for the deposition of insulating layers such as Al2O3 and, furthermore, the method has made it possible to deposit crystalline Al2O3-layers at substrate temperatures in the range of 500 to 800xc2x0 C. In the bipolar dual magnetron system, the two magnetrons alternately act as an anode and a cathode and, hence, preserve a metallic anode over long process times. At high enough frequencies, possible electrical charging on the insulating layers will be suppressed and the otherwise troublesome phenomenon of xe2x80x9carcingxe2x80x9d will be limited. Hence, according to U.S. Pat. No. 5,698,314 the DMS sputtering technique is capable of depositing and producing high-quality, well-adherent, crystalline xcex1-Al2O3 thin films at substrate temperatures less than 800xc2x0 C. The xe2x80x9cxcex1-Al2O3 layersxe2x80x9d with a typical size of the xcex1-grains varying between 0.2 and 2 xcexcm, may partially also contain the gamma (xcex3) phase from the xe2x80x9cxcex3 seriesxe2x80x9d of the Al2O3-polymorphs. The size of the xcex3-grains in the coating is much smaller than the size of the xcex1-grains. The xcex3-Al2O3 grain size typically varies between 0.05 and 0.1 xcexcm. In the Al2O3-layers where both modifications of xcex3 and xcex1-phase were found, the xcex3-Al2O3-phase showed a preferred growth orientation with a (440)-texture. When compared to prior art plasma assisted deposition techniques such as PACVD as described in U.S. Pat. No. 5,587,233, the novel, pulsed DMS sputtering deposition method has the decisive, important advantage that no impurities such as halogen atoms, e.g., chlorine are incorporated in the Al2O3-coating.
It is an object of this invention to avoid or alleviate the problems of the prior art.
It is further an object of this invention to provide a method of depositing at least one layer of fine-crystalline xcex3-Al2O3 by reactive magnetron sputtering technique onto a cutting tool for metal machining.
It is an aspect of the invention to provide a process for producing a coated cutting tool having a coating and a substrate, said coating comprising at least one refractory layer consisting of fine-grained, crystalline xcex3-Al2O3, said process comprising depositing by reactive magnetron sputtering onto a moving substrate in a vacuum by pulsed magnetron sputtering with a mixture of a rare gas and a reactive gas at a pulse frequency set for 10 to 100 kHz, whereby deposition occurs with a rate of at least 1 nm/s with reference to a stationarily arranged substrate, at a magnetron target power density in time average set for at least 10 W/cm2 substrate temperature set in the range of 400 to 700xc2x0 C., and the flux of impinging particles onto the substrate being cyclically interrupted.
According to the present invention, there is provide an improved method of depositing a hard and wear resistant xcex3-Al2O3 layer by pulsed magnetron sputtering at substrate temperatures of 400 to 700xc2x0 C. on a cutting tool for metal machining such as turning (threading and parting), milling and drilling. Said cutting tool comprises a body of a hard material such as cemented carbide, cermets, high speed steel or a superhard material such as cubic boron nitride and/or diamond. The xcex3-Al2O3-layers consist of high-quality, dense, fine-grained crystalline Al2O3 with a grain size less than 0.1 xcexcm and they are virtually free of cracks and halogen impurities.
The xcex3-Al2O3 layer may be included in a wear resistant coating composed of one or more layers of refractory compounds at which the xcex3-Al2O3-layer preferably is the outermost layer and the innermost layer(s), if any at all, between the tool body and the Al2O3-layer, is composed of metal nitrides, carbonitrides and/or carbides with the metal elements selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al.
In contrast to the state of the art, the xcex3-Al2O3-layers according to the present invention further give the cutting edges of the tool an extremely smooth surface finish which, compared to prior art xcex1-Al2O3 coated tool, results in an improved surface finish also of the workpiece being machined. The very smooth surface finish can be attributed to the very fine crystallinity of the coating. The xe2x80x9cxcex3-Al2O3xe2x80x9d-layers may also partially contain other phases from the xe2x80x9cxcex3-seriesxe2x80x9d like xcex8, xcex4 and xcex7.
The fine-grained, crystalline xcex3-Al2O3 according to the present invention is strongly textured in the [440]-direction, A Texture Coefficient, TC, can be defined as:       TC    ⁡          (      hkl      )        =                    I        ⁡                  (          hkl          )                                      I          o                ⁡                  (          hkl          )                      ⁢                  {                              1            n                    ⁢                      ∑                                          I                ⁡                                  (                  hkl                  )                                                                              I                  o                                ⁡                                  (                  hkl                  )                                                                    }                    -        1            
where
I(hkl)=measured intensity of the (hkl) reflection
I0(hkl)=standard of intensity from the ASTM standard powder pattern diffraction data
n=number of reflections used in the calculation
(hkl) reflections used are: (111), (311), (222), (400) and (440) and whenever the TC(hkl) greater than 1, there is a texture in the [hkl]-direction. The larger of the value of TC(hkl), the more pronounced is the texture. According to the presently claimed invention, the TC for the set of (440) crystal planes is greater than 1.5.
When the very fine-grained xcex3-Al2O3 coated cemented carbide cutting tools according to the invention are used in the machining of steel or cast iron, several important improvements compared to prior art have been observed. Surprisingly, the PVD xcex3-Al2O3 without containing any portion of the coarser and thermodynamically stable xcex1-Al2O3-phase, shows, in certain metal machining operations, a wear resistance which is equal to the wear resistance found in coarser CVD xcex1-Al2O3-coatings deposited at temperatures around 1000xc2x0 C. Furthermore, the fine-grained PVD xcex3-Al2O3-coatings show a wear resistance considerably better than prior art PVD-coatings. These observations open up the possibility to considerably improve the cutting performance and prolong the tool lives of coated PVD tools. The low deposition temperature will also make it possible to deposit PVD xcex3-Al2O3-coatings on high-speed steel tools.
A further improvement in cutting performance can be anticipated if the edges of the xcex3-Al2O3 coated cutting tools according to the invention are treated by a gentle wet-blasting process or by edge brushing with brushes based on, e.g., SiC, as disclosed in U.S. Pat. No. 5,861,210.
The total coating thickness according to the present invention varies between 0.5 and 20 xcexcm, preferably between 1 and 15 xcexcm, with the thickness of the non-Al2O3-layer(s) varying between 0. 1 and 10 xcexcm, preferably between 0.5 and 5 xcexcm. The fine-grained xcex3-Al2O3-coating can also be deposited directly onto the cutting tool substrate and the thickness of said xcex3-Al2O3 varies then between 0.5 and 15 xcexcm, preferably between 1 and 10 xcexcm. Similarly, further layers of metal nitrides and/or carbides with the metal elements selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al can be deposited on top of the Al2O3-layer.
The xcex3-Al2O3 layer according to the presently claimed invention is deposited by pulsed magnetron sputtering at a substrate temperature of 400-700xc2x0 C., preferably 500-600xc2x0 C., using aluminum targets and a mixture of at least one rare and at least one reactive gas, preferably argon and oxygen. A preferred solution to carry out the pulsed magnetron sputtering process is the usage of a Dual Magnetron System (DMS). Additionally, the process according to the presently claimed invention is characterized in cyclic interruptions of the flux of particles impinging onto each individual substrate. This flux comprises neutrals, ions, electrons, photons, etc. It seems that these interruptions cause renucleation processes resulting in the observed very fine grained structure of the xcex3-Al2O3 layer. One easy way to realize said cyclic interruptions of the flux is to fixture the substrates on a cylindrical basket which rotates in from of the magnetrons, hence causing the substrates moving in and out of the plasma deposition zone. Alternatively, the cyclic interruptions can also be attained by shadowing the targets or substrate or by interrupting the power supply of the sputtering devices. The frequency of said cyclic interruptions is between 0.1 and 10 per minute. The duration of the interruption of the flux of the impinging particles is at least 10% of the duration of the whole period. Alternatively the cyclic interruption of the flux of impinging particles occurs a periodically. A further characteristic of the process is setting the flow of the reactive gas for such a value that the impedance of the magnetron discharge lies between 150% and 250% of the impedance of a discharge burning between totally oxide-covered target electrodes. This totally oxide-covered state of the targets is indicated by a drastically reduced deposition rate and the presence of oxygen lines in the optical emission spectrum of the plasma. Further, an improvement in the microstructure and phase composition of the xcex3-Al2O3 layer will be achieved by applying a bipolar pulsed bias voltage to the substrates during the deposition. This bipolar bias voltage is preferably asymmetric for both polarities with regard to at least one of the parameters voltage level and pulse duration. This leads to an alternating flux of ions and electrons necessary for the cyclic discharge of the growing insulating layer. Preferred is a bias voltage level between 20 and 200 V, preferably between 50 and 100 V, at frequency in the range of 1-5 kHz. Depending on the geometric conditions of the deposition arrangement, an asymmetric bias pulsing with regard to the parameters voltage level and the pulse duration can be useful. In this case, the duration of the positive polarity should be significantly lower than or at most equal to the duration of the negative polarity. Preferably, the pulse bias frequency lies in the range of 100 Hz to 10 kHz, preferably in the range of 1 kHz to 5 kHz, and the duration of the positive polarity of the substrate is at most equal to, preferably 5 to 20 times lower, than the duration of the negative polarity.
The layer(s) described above, comprising metal nitrides and/or carbides and/or carbonitrides and with the metal elements selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al can be deposited by PVD-technique, CVD- and/or MTCVD-technique (Medium Temperature Chemical Vapor Deposition).